The invention generally relates to cooling a fuel cell stack.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), a membrane that may permit only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is oxidized to produce hydrogen protons that pass through the PEM. The electrons produced by this oxidation travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions may be described by the following equations:
H.sub.2.fwdarw.2H.sup.+ +2e.sup.- at the anode of the cell, and PA1 O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O at the cathode of the cell.
Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several serially connected fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage. The fuel cell stack may include different flow plates that are stacked one on top of the other in the appropriate order, and each plate may be associated with more than one fuel cell of the stack. The plates may be made from a graphite composite or metal material and may include various flow channels and orifices to, as examples, route the above-described reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. The anode and the cathode may each be made out of an electrically conductive gas diffusion material, such as a carbon cloth or paper material, for example.
Referring to FIG. 1, as an example, a fuel cell stack 10 may be formed out of repeating units called plate modules 12. In this manner, each plate module 12 includes a set of composite plates that may form several fuel cells. For example, for the arrangement depicted in FIG. 1, an exemplary plate module 12a may be formed from a cathode cooler plate 14, a bipolar plate 16, a cathode cooler plate 18, an anode cooler plate 20, a bipolar plate 22 and an anode cooler plate 24 that are stacked from bottom to top in the listed order. The cooler plate functions as a heat exchanger by routing a coolant through flow channels in either the upper or lower surface of the cooler plate to remove heat from the plate module 12a. The surface of the cooler plate that is not used to route the coolant includes flow channels to communicate either hydrogen (for the anode cooler plates 18 and 24) or air (that provides the oxygen for the cathode cooler plates 14 and 20) to an associated fuel cell. The bipolar plates 16 and 22 include flow channels on one surface (i.e., on the top or bottom surface) to route hydrogen to an associated fuel cell and flow channels on the opposing surface to route oxygen to another associated fuel cell. Due to this arrangement, each fuel cell may be formed in part from one bipolar plate and one cooler plate, as an example.
For example, one fuel cell of the plate module 12a may include an anode-membrane-cathode sandwich, called a membrane-electrode-assembly (MEA), that is located between the anode cooler plate 24 and the bipolar plate 22. In this manner, upper surface of the bipolar plate 22 includes flow channels to communicate oxygen near the cathode of the MEA, and the lower surface of the anode cooler plate 24 includes flow channels to communicate hydrogen near the anode of the MEA.
As another example, another fuel cell of the plate module 12a may be formed from another MEA that is located between the bipolar plate 22 and the cathode cooler plate 20. The lower surface of the bipolar plate 22 includes flow channels to communicate hydrogen near the anode of the MEA, and the upper surface of the cathode cooler plate 20 includes flow channels to communicate air near the cathode of the MEA. The other fuel cells of the plate module 12a may be formed in a similar manner. To communicate the hydrogen, oxygen and coolant through the stack, the plates include openings that align to form passageways of a manifold.
The fuel cell stack may be located outside of a house that is powered by the fuel cell stack. Due to this arrangement, the coolant (typically de-ionized water) may freeze in colder climates, an event that may disrupt or terminate the generation of power by the fuel cell stack.